A water/steam cycle of a thermal power plant in general comprises—as shown in the schematic diagram of FIG. 1—steam generator 11, a steam turbine 12, a condenser 13 and a feedwater pump 15. The steam generator 11, which may be a heat recovery steam generator HRSG of a combined cycle power plant CCPP, generates steam by heating up feedwater, which is pumped to the steam generator 11 by means of the feedwater pump 15. The generated steam is used to drive the steam turbine 12, which may have high-pressure, intermediate pressure and low pressure stages. The steam, which leaves the steam turbine 12, is converted back into feedwater by means of the water cooled condenser 13 with its internal cooling water circuit 14. In order to keep the water/steam circuit 10 running with good efficiency and without malfunction, it is necessary to permanently remove from the cycle air and/or inert gases, which have entered the cycle through leaks, sealing, and the like. This is usually done by separating those gases from the steam, especially in the condenser 13, and pumping them down, e.g. with an external ejector/vacuum pump.
The configuration of a typical water cooled condenser 13 is shown in FIG. 3 (see the documents CH 423 819, EP 0 325 758 A1, EP 0 384 200 A1 and EP 0 841 527 A2). The condenser 13 comprises within a condenser shell 28 a plurality of separated tube bundles 18, which are arranged in parallel to allow the steam 16 that enters the condenser through an inlet section 17, to come into close thermal contact with the cooling water flowing through the tubes 19 of each tube bundle 18. The condensed steam is collected in a hot well 24 arranged below the tube bundles 18, and then led to the feedwater pump 15.
In the interior of each tube bundle 18 a cavity 20 is provided, which contains an air cooler 21 for finally separating the gases to be pumped down, from the remaining steam. The air coolers 21 are connected to an ejector/vacuum pump 25 via an internal piping 22 and a common suction line 23.
In the prior art, typically, auxiliary steam is used to seal the condenser and electric vacuum pumps are used to evacuate the condenser prior to start-up. However, these components are expensive and unreliable.
On the other hand, if no such additional components are used, suction side pressure losses reduce the performance of the ejector/vacuum pump 25 and substantially increase condenser evacuation time during start-up of the cycle. FIG. 2 shows in a diagram the pressure p as a function of time t during evacuation at the condenser 13 (curve A) and at the entrance of the ejector/vacuum pump 25 (curve B). As one can easily see from the diagram, there is a substantial pressure drop Δp of app. 25% from the condenser 13 to the ejector/vacuum pump 25. As the mass flow for such a pump is roughly proportional to the suction pressure, the evacuation time is inversely proportional to the pressure drop Δp. As a consequence, a pressure drop of 25% gives an evacuation time, which is about 33% longer than without such a drop.
For a condenser of the type shown in FIG. 3, the pressure mainly has to two causes: on one hand, the air coolers 21 have small orifices (e.g. several hundred orifices of 7.5 mm diameter, each), which give a substantial flow resistance. On the other hand, the internal piping 22 of the condenser gives an additional restriction.
Document DE 44 22 344 A1 discloses a condenser which consists of a condensation chamber the bottom of which leads into a collecting chamber and of an additional vacuum chamber arranged at the side of the condensation chamber. The vacuum chamber leads also to the collecting chamber at the bottom and is separated from the condensation chamber by a wall. This wall has a passage for a syphon. The condensation chamber comprises within a condenser shell several tube bundles with an internal air cooler, which is connected to the vacuum chamber via a piping system, which is used to evacuate the condensation chamber from not condensing gas. The vacuum chamber itself is connected via an evacuation line with an external vacuum pump. The syphon forms an open reservoir which collects condensate from into the condensation chamber guided condensing steam. A fast start-up of the condenser is realized by evacuating the condensation chamber through the syphon by means of the vacuum pump. The syphon provides a natural stop of flow once the pressure gradient between the condensation chamber and the vacuum chamber has decreased and normal operation of the condenser has started.
The condenser disclosed in DE 44 22 344 A1 is much more complicated and more expensive than the standard condenser described before.